(1) Field of the Invention
The present invention relates to an optical scanning apparatus which linearly scans an image surface of a photoconductive medium in a main scanning direction by exposing the photoconductive medium to a light beam pattern so as to produce an image on the image surface. The optical scanning apparatus of the present invention is applicable to an optical writing module for use in image forming systems such as digital copiers, printers or facsimiles.
(2) Description of the Related Art
An image forming system, such as digital copier, printer or facsimile, which reproduces an image with a large size, such as A0 A1 size, is known. Generally, the image forming systems of this kind are provided with an optical scanning apparatus. The optical scanning apparatus linearly scans an image surface of a photoconductive medium in a main scanning direction by exposing the photoconductive medium to a light beam pattern so as to produce an image on the image surface. In order to achieve the reproduction of large-size copies, it is needed for the optical scanning apparatus to enlarge the effective scanning range on the image surface so as to cover the desired size.
When it is intended to enlarge the effective scanning range of the optical scanning apparatus, one may encounter some technical restrictions of optical systems in the optical scanning apparatus. Currently, the dominant optical scanning apparatus which is adapted to reproduce large-size copies employs a light-emitting diode (LED) array as the light source. The LED array can be easily configured to match the desired size. However, the LED optical scanning apparatus is bulkier and more expensive than an laser-diode (LD) optical scanning apparatus, and there is no denying that the image quality created by the LED optical scanning apparatus is lower than the image quality created by the LD optical scanning apparatus.
Hence, there is the demand for an LD optical scanning apparatus which is configured in a compact, inexpensive structure and produces a large-size image on an image surface of a photoconductive medium with an improved image quality. The LD emits a laser beam and it is widely used as the light source of the image forming system.
As disclosed in Japanese Laid-Open Patent Applications No.61-11720 and No.6-208066, there is known an LD optical scanning apparatus which linearly scans an image surface of a photoconductive medium in a main scanning direction by exposing the photoconductive medium to a light beam pattern so as to produce a large-size image on the image surface.
In order to ensure an elongated scanning range of the light beam on the image surface of the photoconductive medium so as to cover the desired size (such as A0 A1 size), the above-mentioned optical scanning apparatus utilizes a combination of two optical writing modules. Each of the two writing modules includes a rotary polygonal mirror provided as a rotary deflector, and the two polygonal mirrors are rotated independently of each other when scanning the image surface along the same scanning line. Because of the use of the two polygonal mirrors, it is difficult for the above optical scanning apparatus to synchronize the timings of the rotation of the two rotary deflectors with a suitable accuracy. This causes some distortion in the reproduced image to be produced by the above optical scanning apparatus at connecting portions between the two halves of each main scanning line.
As disclosed in Japanese Laid-Open Patent Application No.8-72308, there has been proposed an LD optical scanning apparatus that is adapted to eliminate the above problem. The optical scanning apparatus of this type employs a combination of two optical writing modules and a synchronizing mechanism. The synchronizing mechanism is provided for synchronizing the timings of the rotation of the two rotary deflectors in the writing modules. The two rotary deflectors are synchronously rotated by the mechanism when scanning the image surface along the same scanning line. However, because of the use of multiple laser beams deflected by the two rotary deflectors, it is difficult to improve the image quality at connecting portions between the two halves of each main scanning line in the image created by the above optical scanning apparatus. Further, the above optical scanning apparatus requires the synchronizing mechanism which will make the entire system configuration bulky and expensive.